Satellite positioning such as the Global Positioning System (GPS) is based on providing a receiver with frequent radio sequences containing Code Division Multiple Access (CDMA) coded data by decoding which the receiver can determine very accurately the radio propagation delay relative to the satellite. With the signals of at least four different satellites, a GPS receiver can determine its position.
When switched on, a GPS receiver will first search for satellites in a so-called acquisition process. In successful acquisition a GPS receiver determines very accurately the frequencies of say, four satellites relative to its own GPS local oscillator or clock and decodes their signals. After the acquisition, the receiver must be able to track the satellites. This involves continually receiving signals from the satellites. Of course, the frequencies on which the satellite signals are received depend on the mutual motion of the satellites and the GPS receiver and hence the GPS receiver needs to compensate for the change to maintain the tracking.
GPS satellites orbit around the globe at some 20 000 km height and hence their radio signals cover a significant surface. Inversely, the power of these signals is very small when received by a GPS receiver at a sea level, particularly compared to the power of the signals that can be received from mobile communication networks, for example. Further, the weaker the GPS signals are, the more difficult it is to find their frequency.
Different crystals are typically used as clocks in GPS receivers. Crystals have a favourable balance of accuracy and price for this use. However, the frequency provided by a crystal depends on its temperature. Even worse, the temperature dependence may not be linear, but may follow a curve that has greatly variable positive, negative and again positive gradients on increasing temperature within the range of −20 to +60 degrees Celsius. At certain temperatures the gradient is negligible and hence some high precision devices contain an oven regulated to keep a constant temperature above the ambient temperature to avoid temperature change originated clock drifting. It is also known to use complex expensive temperature compensated crystals (TCXO) or even more complex Temperature Compensated Voltage Controlled Oscillators (TCVCXO) where the frequency of the clock can be further regulated to remain sufficiently constant. The more advanced these clocks are in technology, however, the more expensive and complex they are. It would be beneficial to use a non-compensated plain crystal as a clock.
Common to the different types of crystals used as GPS receiver clocks, they all have a far lower frequency than that on which the GPS signals are received. A frequency multiplication is thus needed typically by means of a phase locked loop (PLL) and Voltage Controlled Oscillator (VCO). Typically, the GPS receiver clock frequency is 10 to 30 MHz, and conversely a multiplier of roughly 50 to 160 is needed to produce a signal close to the GPS radio frequency that can be used in demodulating the GPS signals. In principle, the accuracy of the demodulation signal should have the same relative accuracy as the GPS receiver clock has, regardless of the frequency of the GPS receiver clock. If the GPS receiver clock is accurate to 1 Part Per Million (ppm) or 1/10 ppm, then respectively the demodulation signal should be accurate to 1 or 1/10 ppm, respectively. In comparison, the clocks used in GPS satellites are very accurate. They use atomic clock references and should have stabilities better than 0.01 ppb (that is 0.00001 ppm).
U.S. Pat. No. 5,841,396 discloses a combined mobile telephone and GPS receiver, wherein a clock of the mobile telephone is used to produce a relatively accurate GPS demodulation signal. The clock of the mobile telephone is also synchronised with a high-precision clock of a base station by using the radio signals received from the base station, based on the fact that the base station couples the frequency of its radio transmission to its high-precision clock that typically has stability in the magnitude of ±0.05 ppm. The mobile telephone frequency stability is generally standardised as better than ±0.5 ppm.